McLean "surprised his girlfriend Rochelle in [club] Wasted Space after running to Rocks the Jeweler inside the Hard Rock Hotel, and buying a beautiful diamond ring," the source says. "He ran back to Wasted Space for the strike of midnight, and the change of date to Jan 9—his birthday!"

It may have been spur-of-the-moment, but that doesn't make it any less romantic. "He brought Rochelle up to the middle of the stage, stops the music and proposed. She said yes and the crowd went crazy," the source gushed.

The newly betrothed duo plan to continue the birthday , and now engagement, celebration tonight at the Hard Rock's club, Vanity.

As for the rest of the Boys, their seventh full length album hit shelves back in October, and a second greatest hits album, Playlist: The Very Best of the Backstreet Boys, is set for a Jan. 26, 2010 release.